It is the middle of a neck feather that is used. The very tip of the feather cannot be wrapped around the hook because it is the end you hold on to with the hacklepliers. And the butt end of the feather cannot be used as hackle because it the part with the most web. This is why it is the middle of the feather that gets measured.

Keep in mind that if you are tying with dry fly saddle hackle, the hackle is virtually the same barb length the whole length of the feather. Therefore, with dry fly saddles, measure is anywhere. And if you get in the habbit of measuring the middle of the feather, it works for both neck and saddle hackle.